The main industrial process for producing nitric acid is known as the Ostwald route where ammonia, NH3 is oxidised over a combustion catalyst at temperatures in the range of 800 to 1100° C. to form nitrogen monoxide, NO. The pressure ranges from atmospheric pressure to 10-12 bar. The formed nitrogen monoxide is quenched, mixed with air to form nitrogen dioxide, NO2, and then the nitrogen dioxide is allowed to react with water to form nitric acid, HNO3.
The typical combustion catalyst is one or more stacked gauzes made of woven or knitted wires of platinum alloyed with rhodium, and traces of grain refining elements. During operation the combustion catalyst loses platinum and to a lesser extent rhodium via the volatisation of PtO2 and RhO2. Thus it is common industrial practice to place catchment gauzes downstream of the combustion gauzes in order to recover a proportion of the platinum loss.
Prior Art
The current PGM-catchment technology is based on palladium or palladium based alloys (palladium, containing small amounts of platinum, and silver, gold, cobalt or nickel). The catchment alloy is installed in the form of gauzes, directly downstream of the platinum-based combustion gauzes. Typically, the catchment gauze is produced from 60 to 90 micron wire and is woven with a 1024 mesh (32 wires per linear cm), and 3 or 4 catchment gauzes would be installed.
The benefits of installing the catchment alloy in the form of a gauze are two-fold. In terms of their production, the catchment systems are made using the same technology as the combustion gauzes. The production method is therefore well known and proven. The main technological advantage of a gauze-based catchment system is the very high mass transfer characteristics of gauzes.
However, installing the catchment alloy in the form of gauzes has a number of disadvantages. Producing gauzes is a relatively expensive production process (producing wire and weaving or kitting the wire). To aid wire production the palladium is typically alloyed with a base metal, which evaporates in operation and thus continuously transports metal into the plant boilers). The surface area of a typical gauze (76 micron-1024 mesh) is relatively low (initially 1.5 cm2 of metal per cm2 of gauze). As the total catchment zone is narrow (0.4 to 0.6 mm) all the recovered platinum is located within this zone. Therefore, during a campaign, the openings in the catchment gauzes become progressively blocked. This leads to a large increase in pressure drop across the pack. A further problem related to the catchment gauzes is that the platinum -palladium alloy, that is formed during the catchment process, becomes embrittled. This, combined with the additional mechanical load on the gauzes, caused by the increased pressure drop, may lead to physical breakdown of the gauzes.
Objective of the Invention
The main objective of this invention is to provide a method and device for catching platinum group metals in a gaseous stream that solves the above-mentioned problems.
A further objective is to provide a method for manufacturing the device according to the invention.
The objectives of the invention may be obtained by the features set forth in the following description of the invention and/or in the appended claims.
Description of the Invention
The invention utilises the realisation that gaseous streams through porous ceramic structures are substantially turbulent and that porous materials have relatively huge surface areas allowing excellent contact area between the flowing gas and the ceramic wall material, which leads to the favourable combination of high mass transfer coefficient of gaseous components onto the ceramic wall material and a huge surface area.
In a first aspect, the invention thus relates to a method for catching platinum group elements in a gaseous stream by passing the gas through a porous ceramic body coated with one or more PGM-catching metal(s) and/or alloy(s). The porous ceramic body may advantageously have all surface area covered by the one or more PGM-catching metal(s) and/or alloy(s) and the applied gas pressure may advantageously result in turbulent gas streams flowing through the channels in the ceramic body.
In a second aspect, the invention relates a device for carrying out the method according to the first aspect of the invention. Thus the second aspect of the invention relates to a porous ceramic body having at least a part of its surface area covered by one or more PGM-catching metal(s) and/or alloy(s). The ceramic body should may advantageously be a ceramic foam or sponge. Suitable ceramics may advantageously be one or more of the following: zirconia, alumina, alumino -silicate, but may also be of any ceramic or metallic material which may be coated with one or more metallic catalyst material(s) and which exhibits the necessary mechanical strength and chemical properties to withstand the conditions encountered in ammonia burners. Thus, the ceramic body can be made of one or more of the following materials: zirconia, alumina, alumino-silicate or a refractory oxide, silicate, carbide, boride, phosphate, nitride or a refractory metal. The porosity of the ceramic body may advantageously be in the range of 50 to 98% with a preferred pore size, defined by the number of pores per linear inch, in the range of 5 to 120 ppi. This corresponds to about 2 to 50 pores per cm. The deposited layer of one or more PGM-catching metal(s) and/or alloy(s) may have a thickness in the range from about 1 nm up to about 150 micron.
The surface area of ceramic foams according to the invention is comparable to that of a widely used monolith structure, with the same characteristic dimension (channel size and pore size). However, the mass transfer coefficient for a sponge of foam is comparable with that of wire gauzes, which is significantly higher than of a monolith, as turbulent flow is present throughout the depth of the sponge. Thus the invention provides a support system having a contact area greater than can be practically achieved with gauzes, but with a comparable mass transfer coefficient.
Currently, the most common catchment alloys are based on palladium, with the addition of alloying components to improve the wire drawing, weaving or knitting properties. Examples of the systems include Pd—Au, Pd—Co and Pd—Ni binary alloys. Trace quantities of grain refining elements may also be present.
A the third aspect of the invention relates to a method for depositing the one or more PGM-catching metal(s)/alloy(s) onto the ceramic foam. An advantageous method is electroless plating, which is an autocatalytic coating method that allows both electrically conducting and insulating materials to be coated with a uniform metallic layer. In this method the ceramic object to be coated is activated by sequentially dipping into a tin solution and a palladium solution. The tin is adsorbed onto the surface as a Sn2+ species, in sub-monolayer quantities. When the palladium contacts the Sn2+, it will become reduced to metallic palladium:Sn2++Pd2+→Sn4++Pdo 
After the activation process, the ceramic sponge is placed in a solution containing the metal that is to be deposited, which in our application is palladium, along with a reducing agent, such as hydrazine. The metallic palladium on the surface acts as a catalyst for the reduction of more palladium, by the hydrazine. By this means, a uniform layer of metallic palladium is deposited onto the surface. The coating thickness may be controlled by varying the coating time and the solution chemistry.
The palladium coating is deposited onto the surface of the support sponge via an electroless plating technique. The process involves an activation step and a coating step. The activation step consisted of the immersion of the sponge into a solution of tin (II) chloride; rinsing with deionised water and immersion into a solution of palladium chloride. The activation step was repeated between 5 and 10 times. During the activation process, the colour of the sponge changes to a pale grey colour. An example of the composition of an activation solution is shown in Table 1.
TABLE 1An example of the compositions of an activation solutionSnCl2•2H2O1g/lHCl (37%)1ml/lTemperature25°C.PdCl20.1g/lHCl (37%)1Temperature25°C.
After the activation of the sponge, it may be coated with palladium. The coating solution consists of an aqueous solution of palladium tetra-amine dichloride (Pd(NH3)4Cl2.4H2O, which had been complexed with disodium ethylenediaminetetraacetic acid dehydrate (Na-EDTA), along with ammonium hydroxide, for a minimum of 12 hours. Just prior to the coating operation, the solution was heated to 60° C., and then hydrazine was added to the coating solution. An example of the composition of a coating solution is shown in Table 2. The coating solution is contacted with the sponges in a flow system, such that the solution flows through the sponge, or a series of sponges and after passing through them, is recycled. The solution is passed through the sponges for between 10 minutes and 1 hour.
TABLE 2An example of the composition of a coating solutionPd(NH3)4Cl2•H2O4g/lNa2-EDTA•2H2O40.1g/lNH4OH (28%)198Metal concentration16.3mMPh10-11Temperature60°C.
The thickness or palladium loading on the sponge is readily controlled by the temperature and time of the coating process. High temperatures and long time favour thicker coatings. If a thick layer of palladium is required, the coating solution may be replaced after a period of time.